This invention relates to the field of vehicle counting. More specifically, this invention relates to techniques for counting moving vehicles.
The need to provide an accurate count of the number of vehicles passing by a selected location has existed for a substantial period of time. Vehicle count information is used for a number of purposes, such as determining the total volume of vehicular traffic through a particular intersection or past a given location of a highway. In the past, vehicle counting has been effected in a number of ways. Perhaps the most popular way has been to hire an individual to stand at the particular location and actually count the number of vehicles observed by that individual passing by the particular location. This method suffers from the disadvantage that the total count obtained, particularly over a relatively long period of time, can be highly inaccurate, depending on the dedication and concentration powers of the individual. Further, the individual is frequently exposed to physical danger due that person's presence at the counting site. Another technique used in the past for counting vehicles employs a road tube placed across one or more lanes of the highway and connected to a compatible counting mechanism. This arrangement suffers from the disadvantage that the road tube is This arrangement suffers from the disadvantage that the road tube is susceptible to physical wear caused by the passage of the vehicle wheels over the tube and direct damage from snow removal equipment, and is also subject to deterioration caused by environmental exposure over severe temperature ranges. In addition, such devices are typically electrically powered, which requires either a permanent or portable source of electrical power which must be reliable over the counting period. In addition, such equipment is prone to tampering and/or theft and can be difficult to install in certain locations. In addition, the road tube counting mechanisms may be expensive to purchase and repair.
Vehicle detector systems have been used for a substantial period of time to generate information specifying the presence or absence of a vehicle at a particular location. Such detectors have been used at intersections, for example, to supply information used to control the operation of the traffic signal heads and have also been used to supply control information used in conjunction with automatic entrance and exit gates in parking lots, garages and buildings. Since the purpose for which vehicle detector systems have been developed requires only the determination of whether a vehicle of a particular class (i.e., size or weight) is present or absent, such systems are not directly suitable for use in counting the total number of vehicles passing by a specified location. For example, in an application for a controlled intersection having a left turn green arrow lane, the green arrow may be controlled in such a manner that activation is only done when a vehicle is actually present in the left turn lane. Such presence is indicated by a change in inductance of a closed loop circuit driven by an oscillator in the vehicle detector system, the inductance decreasing from a reference value when a vehicle enters the loop (or is in close proximity to the loop). So long as this changed level of inductance remains, the left turn lane vehicle detector will signal the presence of a vehicle in that lane by generating a signal termed a Call Signal. When the green arrow is activated by the traffic control system, the vehicle originally present in the loop and waiting for permission to enter the intersection leaves the loop. If this were the only vehicle in the loop, then the inductance changes back to a value close to the reference value and the traffic control unit is then free to time out the permissive green signal. If more than one vehicle was originally present in the left turn lane and thus affecting the inductance of the loop circuit, the vehicle detector will simply continue to register the call signal until the last vehicle has left the loop (or until the system has reached a maximum time out limit). As a consequence of this design, vehicle detector systems have not been capable, as originally designed, of providing an accurate count of the total number of vehicles crossing the loop. Since a large number of vehicle detectors are already installed in highways, and since the vehicle detector system technology in general has reached a relatively high degree of sophistication, it would be most beneficial if such systems could be adapted to provide a vehicle counting function without expensive additions to the raodway loop system.